The present invention relates to a method of correcting defects in uniformity of a tire and, more particularly, it relates to a method which makes it possible to reduce the variations of force in tires, said variations producing a number of unfavorable effects in the driving of a vehicle, such as various vibrations, poor straight-line stability of the vehicle, comfort, low level and abnormal feelings in steering.
These defects in uniformity have numerous causes, and it is very difficult to build a perfectly uniform tire in view of the numerous variables which exist. Some of these variables can be enumerated: dimensional irregularities of the semi-finished products entering into the formation of the tire, irregularities in weight of the products themselves, irregularities in the composition and physical properties, irregularities due to the methods of building the tire, such as variations in thickness due to overlap, irregularities due to the vulcanization of the tire, such as variations in vulcanization temperature or variations in position in the vulcanization mold.
The uniformity of a tire is generally measured by the degree of perfection with regard to its physical dimensions, the weight balancing and the variations in dynamic forces. Numerous methods have been used in order to attempt to correct or compensate for each of the irregularities observed. It is known, for instance, to incorporate small amounts of rubber in the crown and in the inside of the tire in order to balance the tire. The compensating for the imbalance of the assembly consisting of tire plus wheel can also be effected by adding small weights on the wheel rim. Furthermore, it is current practice to correct the dimensions of a tire by grinding the tread thereof so as to make it perfectly round and concentric with respect to its axis of rotation.
Measurements of the variations in dynamic forces are at present simple and are carried out on machines, known as "uniformity" machines, which permit the inflated tire under load, mounted on a rotating axle, to travel on a drum the shaft of which is coupled to a relatively complicated system of suitable stress gauges. Means for measuring the forces exerted on these gauges are provided and the signals produced by these forces are transmitted to a reading/recording device.
Numerous tests on vehicles have shown that the variations in radial forces, that is to say in the forces acting perpendicular to the surface of contact between the tire and the ground, were by far the most harmful with regard to the unfavorable effects on the driving and road-holding characteristics of the vehicles. As is known, the recorded curve of the variation in radial force (F.sub.R) upon one revolution of the wheel is comparable to the curve representative of a vibratory movement, which can be broken down into a sum of harmonic sinusoidal movements by means of harmonic analyzers. The responsibility for the irregularities produced in the behavior of the vehicle is customarily attributed to the variations of the first harmonic (F.sub.RH1) and the second harmonic (F.sub.RH2) since generally they are the most important in amplitude and have the most disturbing frequencies. However, harmonics of a higher order can prove disturbing for certain vehicles in view of their frequencies and therefore the strict control carried out on tires and rolling assemblies, not only as to the variation in total radial force but also as to the harmonics which result therefrom. In general, harmonics of a higher order than 16 are insignificant and are not taken into account.
U.S. Pat. No. 3,724,137 describes a manner of attenuating the variations in radial forces. In combination with the measurement machine, two rotary grinding wheels are placed in positions adjacent to the shoulders of the tread of the tire so that these grinding wheels remove material from the ribs of the shoulders of the tread as a function of the variations in radial forces detected on the drum and measured until these variations have been reduced to a desired acceptable level. Numerous patents are characterized by the use of such a grinding of the tread.
Modifying the thicknesses of the tread by grinding would not appear to be the best solution to contemplate for correcting these variations in dynamic forces since the tread is, in fact, an essential element of the tire as it assures contact with the ground and numerous desired qualities of the tire depend on this contact.